Re: [PATCH v5 00/21] Virtual Swap Space

[Christoph Hellwig]

On Sat, Apr 11, 2026 at 06:40:44PM -0700, Nhat Pham wrote:
> > However, if the modularization from point 1 is achieved and
> > vswap acts as a swap device itself, then we can cleanly
> > establish a:
> >
> >   virtual -> physical
> 
> I read that thread sometimes ago. Some remarks:
> 
> 1. I think Christoph has a point. Seems like some of your ideas ( are
> broadly applicable to swap in general. Maybe fixing swap infra
> generally would make a lot of sense?

I think a first step would be a dump of that code, even if it is against
an old kernel so that everyone knows what we are talking about.

> 2. Why do we need to do two virtual layers here? For example, If you
> want to buffer multiple swap outs and turn them into a sequential
> request, you can:
> 
> a. Allocate virtual swap space for them as you wish. They don't even
> need to be sequential.
> 
> b. At swap_writeout() time, don't allocate physical swap space for
> them right away. Instead, accumulate them into a buffer. You can add a
> new virtual swap entry type to flag it if necessary.
> 
> c. Once that buffer reaches a certain size, you can now allocate
> contiguous physical swap space for them. Then flush etc. You can flush
> at swap_writeout() time, or use a dedicated threads etc.

That matches what file systems do with delalloc, where space 2 just
adjust an in-memory counter for space reservations.

> Deduplication sounds like something that should live at a lower layer
> - I was thinking about it for zswap/zsmalloc back then. I mean, I
> assume you don't want content sharing across different swap media? :)
> Something along the line of:

Does dedup in swap really make much sense?  If you want to dedup you
also want to do that in-memory, i.e. using ksm.